Water-level fluctuations affect the alpha and beta diversity of macroinvertebrates in Poyang Lake, China

Water-level fluctuations (WLFs) are a key influence on aquatic biodiversity in seasonally inundated freshwater ecosystems. However, how unregulated WLFs affect macroinvertebrates in lake-floodplain systems experiencing considerable annual fluctuations remains unclear. We explored spatial and temporal variability in taxonomic alpha and beta diversity in the largest fluctuating lake in China, Poyang Lake, during two hydrological seasons. We hypothesized that taxa richness (alpha diversity) is greater in the floodplain than in the lake channel due to greater availability of trophic resources, and that variability in assemblage composition (beta diversity) in the channel is greater during the high-water season (HWS) than the low-water season (LWS) due to increased habitat heterogeneity. Benthic macroinvertebrate assemblages were sampled, water physicochemical and hydrological variables were measured, and geographical coordinates were determined at 34 sites during the HWS (October 2017) and LWS (January and April 2018). A total of 74 taxa were recorded. Macroinvertebrate alpha diversity was comparable in the floodplain and the lake channel. Beta diversity in the channel was greater during HWS than LWS. Hydrological variables influenced beta diversity during LWS and geographical distance between sites increased beta diversity during HWS, whereas physicochemical variables did not influence beta diversity in either hydrological season. Our results suggest that extensive WLFs altered macroinvertebrate biodiversity among hydrological seasons by extending water into floodplains during HWS and reducing substrate heterogeneity in the lake channel during LWS. We thus highlight the importance of WLFs that maintain such environmental seasonality in supporting the biodiversity of benthic macroinvertebrates in naturally dynamic freshwater ecosystems.